Returns a new DSA instance that carries just the
public key information. If the current instance has also private key
information, this will no longer be present in the new instance. This
feature is helpful for publishing the public key information without
leaking any of the private information.

Computes and returns the DSA signature of
string, where string is expected to be an
already-computed message digest of the original input data. The signature
is issued using the private key of this DSA
instance.